In the manufacturing of many electronic and opto-electronic products, it is necessary to fabricate millions of microscopic structures on a single large substrate. The structures can be in the form of active devices, such as the transistors in an electronic display or a semiconductor integrated circuit, or in the form of passive patterns, such as the metal interconnect network in a multichip packaging module or a printed circuit board. The large substrate can be a display panel, a silicon wafer, or a board. The pattern feature sizes in these diverse products, which we shall call electronic modules, range from sub-micron for semiconductor chips to multi-microns for displays and packaging products. The substrate size requirements vary from a few square inches for small modules to a few square feet for large displays.
FIG. 1 describes the prior art and schematically illustrates how one type of lithography system is used to transfer the pattern from a mask onto a substrate to expose a single module. The substrate 10 and the mask 14 are held rigidly in a substrate stage 12 and a mask stage 16, respectively. Both the substrate and the mask stages move in synchronism with fine precision. The illumination system 18 consists of a source system 20, a relay lens 22, and beam steering optics 24. The relay lens collects radiation emitted from the effective emission plane of the source into a certain numerical aperture, NA.sub.S, and directs it with a certain magnification and numerical aperture, NA.sub.C, on to the mask. A projection lens assembly 26, which may consist of several individual lens elements and prisms or mirrors, forms a precise image of the high-resolution pattern contained within the illuminated region on the mask on to the substrate. The projection lens has a numerical aperture NA determined by the resolution requirements of the patterning system and is designed for as large an image field as possible.
The substrate stage 12 scans the substrate across its exposure region so as to traverse the length of the substrate in the direction of the scan. Simultaneously, the mask stage 16 scans the mask across its illuminated region. After completion of a scan, both stages move in a direction orthogonal to the scan direction by an amount termed the `effective scan width.` Following such a lateral movement, a new scan is generated by precise movements of the substrate and mask stages in the same manner as before. The above exposure process is repeated until the entire substrate is exposed. A control system 30 is functionally coupled to the illumination system, the mask and substrate stages, and the projection lens assembly, and ensures that the mask and substrate stages are focused and aligned appropriately with respect to the projection lens assembly at all times, that the mask and substrate stages perform the scan and repeat movements with the desired synchronism, and that the illumination system maintains the desired illumination characteristics throughout the exposure of the entire substrate.
In large area lithography it is often necessary to expose the same pattern onto different areas, called modules, of a substrate. The first module is exposed by scanning the mask and the substrate as described above. After the exposure is complete, it is then necessary to align the second module area with the mask to reproduce the same pattern on the second module. This can be done by moving only the substrate--also referred to as indexing the substrate--so that the second area is in the patterning region of the projection lens of the lithography system. Indexing the substrate implies that the substrate stage must move the substrate by an amount equal to the size of the module width or length.
In the exposure process described above, the substrate stage must have a range of travel that is large enough to cover the module area in order to scan the substrate in unison with the mask stage during exposure. In addition, to index the substrate from one module area to the next, the substrate stage must also have a range of travel that is large enough to index to all of the module areas and scan or step with the appropriate precision to expose all of the areas of the substrate.
One of the inventors, K. Jain, has previously patented a variety of large-area patterning systems (U.S. Pat. Nos. 4,924,257; 5,285,236 and 5,291,240). These previous patents disclosed projection imaging apparatus for producing very high-resolution patterns for integrated-circuit fabrication on a silicon-wafer-size (a few tens of sq. inches) substrate using reduction patterning with a mask that is larger than an individual integrated circuit, and apparatus for producing patterns on a large, display-panel-size (a few hundred sq. inches) substrate using 1:1 patterning with a mask that is of the same size as the substrate. Due to reasons of economies of scale, there are many other applications, such as multi-chip module fabrication, which would benefit by a high-throughput, 1:1 projection patterning system on a large substrate capable of accommodating multiple modules. However, 1:1 imaging systems require a mask that is of the same size as the substrate. The high cost of large masks is a disadvantage of 1:1 patterning systems.
Thus, it is highly desirable to develop apparatus and method to exploit the benefits of 1:1 large-area projection patterning, using a mask that is significantly smaller than the substrate, which is patterned in modules.
A previous patent application by the same inventor describes such an apparatus, which features an auxiliary stage, to provide repositioning capability for mask or substrate, for sequential 1:1 projection patterning of each substrate module by scanning the mask and substrate simultaneously by a unitary mask-substrate x-y stage. The function of the auxiliary stage is to keep the appropriate part of the mask and/or substrate in the object/image field of view of the projection lens.
These systems require an additional auxiliary stage for indexing the substrate from one exposure area to the next (K Jain, patent pending). Since the indexing stage does not need to move during the exposure, it can be of lower precision and accuracy than the scanning stage; however, it does require that the scanning stage be capable of bearing the extra payload.
This invention frees the size of the substrate panel to be exposed from limitations imposed by the travel range of the scanning stage or the size of the mask. This is accomplished without the use of additional stages or imposing additional requirements on the scanning stage.
Kosugi et al. U.S. Pat. No. 4,775,877, METHOD AND APPARATUS FOR PROCESSING A PLATE-LIKE WORKPIECE, 1988, shows a technique, using a rotatable "hand" for picking up a wafer from a positioning stage, rotating the wafer a half turn, and replacing the wafer on a half-wafer stage for further exposure processing, to double the area of exposure on the wafer.